Electron heating modes and frequency coupling effects in dual-frequency capacitive CF4 plasmas

Two types of capacitive dual-frequency discharges, used in plasma processing applications to achieve the separate control of the ion flux, Гi, and the mean ion energy, , at the electrodes, operated in CF , i4 are investigated by particle-in-cell simulations: (i) In classical dual-frequency discharges, driven by significantly different frequencies (1.937 MHz + 27.12 MHz), and Гi are controlled by the voltage amplitudes of the low-frequency and high-frequeny components, ΦLF and ΦHF, respectively. (ii) In electrically asymmetric (EA) discharges, operated at a fundamental frequency and its second harmonic (13.56 MHz + 27.12 MHz), ΦLF and ΦHF control Гi, whereas the phase shift between the driving frequencies, θ, is varied to adjust . We focus on the effect of changing the control parameter for on the electron heating and ionization dynamics and on Гi. We find that in both types of dual-frequency strongly electronegative discharges, changing the control parameter results in a complex effect on the electron heating and ionization dynamics: in classical dual-frequency discharges, besides the frequency coupling affecting the sheath expansion heating, additional frequency coupling mechanisms influence the electron heating in the plasma bulk and at the collapsing sheath edge; in EA dual-frequency discharges the electron heating in the bulk results in asymmetric ionization dynamics for values of θ around 45°, i.e., in the case of a symmetric applied voltage waveform, that affects the dc self-bias generation

On the self-excitation mechanisms of plasma series resonance oscillations in single- and multi-frequency capacitive discharges

The self-excitation of plasma series resonance (PSR) oscillations is a prominent feature in the cur- rent of low pressure capacitive radio frequency discharges. This resonance leads to high frequency oscillations of the charge in the sheaths and enhances electron heating. Up to now, the phenomenon has only been observed in asymmetric discharges. There, the nonlinearity in the voltage balance, which is necessary for the self-excitation of resonance oscillations with frequencies above the applied frequencies, is caused predominantly by the quadratic contribution to the charge-voltage relation of the plasma sheaths. Using Particle In Cell/Monte Carlo collision simulations of single- and multi-frequency capacitive discharges and an equivalent circuit model, we demonstrate that other mechanisms, such as a cubic contribution to the charge-voltage relation of the plasma sheaths and the time dependent bulk electron plasma frequency, can cause the self-excitation of PSR oscillations, as well. These mechanisms have been neglected in previous models, but are important for the theoretical description of the current in symmetric or weakly asymmetric discharges

Striations in electronegative capacitively coupled radio-frequency plasmas: analysis of the pattern formation and the effect of the driving frequency

Self-organized striated structures of the plasma emission have recently been observed in capacitive radio-frequency CF4 plasmas by Phase Resolved Optical Emission Spectroscopy (PROES) and their formation was analyzed and understood by Particle in Cell / Monte Carlo Collision (PIC/MCC) simulations [Y.-X. Liu, et al. Phys. Rev. Lett. 116, 255002 (2016)]. The striations were found to result from the periodic generation of double layers due to the modulation of the densities of positive and negative ions responding to the externally applied RF potential. In this work, an in-depth analysis of the formation of striations is given, as well as the effect of the driving frequency on the plasma parameters, such as the spatially modulated charged species densities, the electric field, and the electron power absorption is studied by PROES measurements, PIC/MCC simulations, and an ion-ion plasma model. The measured spatio-temporal electronic excitation patterns at different driving frequencies show a high degree of consistency with the simulation results. The striation gap (i.e., the distance between two ion density maxima) is found to be inversely proportional to the driving frequency. In the presence of striations the minimum(CF_3^+, F^-) ion densities in the bulk region exhibit an approximately quadratic increase with the driving frequency. For these densities, the eigenfrequency of the ion-ion plasma is near the driving frequency, indicating that a resonance occurs between the positive and negative ions and the oscillating electric field inside the plasma bulk. The maximum ion densities in the plasma bulk are found not to exhibit a simple dependence on the driving frequency, since these ion densities are abnormally enhanced within a certain frequency range due to the ions being focused into the "striations" by the spatially modulated electric field inside the bulk region.Comment: 31 pages, 16 figure

Striations in electronegative capacitively coupled radio-frequency plasmas: effects of the pressure, voltage, and electrode gap

Capacitively coupled radio-frequency (CCRF) CF_4 plasmas have been found to exhibit a self-organized striated structure at operating conditions, where the plasma is strongly electronegative and the ion-ion plasma in the bulk region (largely composed of CF_3^+ and F^- ions) resonates with the excitation frequency. In this work we explore the effects of the gas pressure, the RF voltage, and the electrode gap on this striated structure by Phase Resolved Optical Emission Spectroscopy and Particle-In-Cell/Monte Carlo Collisions simulations. The measured electronic excitation patterns at different external parameters show a good general agreement with the spatio-temporal plots of the ionization rate obtained from the simulations. For a fixed driving frequency the minima of the CF_3^+ and F^- ion densities (between the density peaks in the bulk) are comparable and independent of other external parameters. However, the ion density maxima generally increase as a function of the pressure or RF voltage, leading to the enhanced spatial modulation of plasma parameters. The striation gap (defined as the distance between two ion density peaks) is approximately inversely proportional to the pressure, while it exhibits a weak dependence on the RF voltage and the electrode gap. A transition between the striated and non-striated modes can be observed by changing either the pressure or the RF voltage; for 13.56 MHz and 18 MHz driving frequencies we present a phase diagram as a function of the pressure and voltage amplitude parameters.Comment: 32 pages, 18 figures. arXiv admin note: text overlap with arXiv:1703.0588

Control of plasma properties via the electrical asymmetry effect

Diese Arbeit beschäftigt sich mit der Kontrolle der Eigenschaften von kapazitiv gekoppelten Plasmen über den Elektrischen Asymmetrie-Effekt (EAE). Der Hauptteil besteht aus der Untersuchung von elektrisch asymmetrischen Wasserstoff- bzw. prozessrelevanten Wasserstoff-Silan-Plasmen. Die experimentellen Ergebnisse zeigen, dass ein Wechsel des Elektronen-Heizungsmodus stattfindet. Ein analytisches Modell beschreibt die Leistungseinkopplung unter Silanbeimischung. Mit dem EAE lässt sich der Einfluss von Stehwelleneffekten kompensieren und die Homogenität des Ionenflusses auf die Elektroden deutlich verbessern. Der DC self-bias, die Randschichtspannungen und damit die Energie der Ionen lassen sich ebenfalls über den EAE kontrollieren. Außerdem wird in Experiment und Modell gezeigt, wie sich die Verteilung von Staubteilchen in Argon-Plasmen mit dem EAE gezielt manipulieren lässt. Ferner wird dargestellt, wie der EAE in schwach elektronegativen Sauerstoff-Plasmen genutzt werden kann.This work is about the control of the properties of capacitively coupled plasmas via the Electrical Asymmetry Effect (EAE). The main part deals with the investigation of electrically asymmetric hydrogen plasmas or process relevant hydrogen silane plasmas. The experimental results show that the dominant electron heating mode changes. An analytical model describes the power dissipation if silane is admixed. Using the EAE, the influence of standing waves at higher applied frequencies can be compensated and the ion flux homogeneity onto the electrodes is significantly improved. The DC self-bias, the sheath voltages, and, accordingly, the ion energy is controlled via the EAE. Moreover, the distribution of dust particles injected into argon plasmas can be manipulated via the EAE, as it is shown experimentally and by a model. Further, it is demonstrated how the EAE can be used in weakly electronegative oxygen plasmas

Electron heating modes and frequency coupling effects in dual-frequency capacitive CF4 plasmas

Two types of capacitive dual-frequency discharges, used in plasma processing applications to achieve the separate control of the ion flux, Гi, and the mean ion energy, , at the electrodes, operated in CF4, are investigated by particle-in-cell simulations: (i) In classical dual-frequency discharges, driven by significantly different frequencies (1.937 MHz + 27.12 MHz), and Гi are controlled by the voltage amplitudes of the low-frequency and high-frequeny components, ΦLF and ΦHF, respectively. (ii) In electrically asymmetric (EA) discharges, operated at a fundamental frequency and its second harmonic (13.56 MHz + 27.12 MHz), ΦLF and ΦHF control Гi, whereas the phase shift between the driving frequencies, θ, is varied to adjust

Electron heating and control of ion properties in capacitive discharges driven by customized voltage waveforms

We investigate the electron heating dynamics in capacitively coupled radio frequency plasmas driven by customized voltage waveforms and study the effects of modifying this waveform and the secondary electron emission coefficient of the electrodes on the spatio-temporal ionization dynamics by particle-in-cell simulations. We demonstrate that changes in the electron heating dynamics induced by voltage waveform tailoring strongly affect the dc self-bias, the ion flux, i, and the mean ion energy, ⟨Ei⟩, at the electrodes. The driving voltage waveform is customized by adding N consecutive harmonics (N 4) of 13.56 MHz with specific harmonics’ amplitudes and phases. The total voltage amplitude is kept constant, while modifying the number of harmonics and their phases. In an argon plasma, we find a dc self-bias, η, to be generated via the electrical asymmetry effect for N 2. η can be controlled by adjusting the harmonics’ phases and is enhanced by adding more consecutive harmonics. At a low pressure of 3 Pa, the discharge is operated in the α-mode and ⟨Ei⟩ can be controlled by adjusting the phases at constant i. The ion flux can be increased by adding more harmonics due to the enhanced electron-sheath heating. ⟨Ei⟩ does not remain constant as a function of N at both electrodes due to a change in η. These findings verify previous results of Lafleur et al. At a high pressure of 100 Pa and using a high secondary electron emission coefficient of γ = 0.4, the discharge is operated in the γ -mode and mode transitions are induced by changing the driving voltage waveform. Due to these mode transitions and the specific ionization dynamics in the γ -mode, i is no longer constant as a function of the harmonics’ phases and decreases with increasing N